Problem: Solve for $r$ : $-23 = -15 + r$
Add $15$ to both sides: $ \begin{eqnarray} \\ \hphantom{-23 {+ 15}} &\hphantom{=}& \hphantom{r} \\ -23 &=& -15 + r \\ \\ {+15} && {+15} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ -23 {+ 15} &=& r \\ \end{eqnarray} $ Simplify. $ r = -8$